1. Field of the Invention
The present invention relates, in general, to a method for fabricating a micro actuator using photolithography and, more particularly, to an improvement in the aspect ratio of piezoelectric/electrostrictive patterns and in their bonding properties to a substrate, along with the method.
2. Description of the Prior Art
Generally, a micro actuator has a stacked structure in which a vibrating plate underlays a piezoelectric/electrostrictive film with two electrodes attached respectively to its bottom and top. If an electric field is applied across the two electrodes, the piezoelectric/electrostrictive film which lies between the two electrodes is repetitively distorted and restored, effecting vibration.
Conventionally, a piezoelectric/electrostrictive film, serving as a piezoelectric element in a micro actuator, is prepared from a ceramic powder produced by a solid phase method. This method, called an oxide method, comprises thermally treating a mixture of raw material oxide powders or their molten salts at, for example, 800-1,200.degree. C., pulverizing and sintering them to yield ceramic powders.
Depending on the particle size of the raw material powder, the ceramic powders produced by this conventional solid phase method have different particle sizes, which are relatively large ranging from 0.2 to 2 .mu.m, so that the solid phase method is unsuitable to produce ceramic particles as fine in size as 0.1 .mu.m or less. In addition, this conventional method is disadvantageous in that a thermal treatment is conducted at a high temperature of 1000.degree. C. or above.
Micro actuators are usually fabricated using a screen printing technique in which a ceramic paste is screen-printed on an infrastructure consisting of a vibrating plate and a chamber, to form fine patterns. In detail, a ceramic paste is printed on a vibrating plate with the aid of a screen and subjected to debindering, followed by sintering the ceramic at 1,000.degree. C. or higher to form a piezoelectric/electrostrictive film with a predetermined thickness.
The screen printing method is widely used by virtue of its ability to increase the integration degree of micro actuators, but suffers from disadvantages as follows:
First, the piezoelectric/electrostrictive film is printed to a limitative low thickness. The thickness with which the conventional screen printing method can endow the piezoelectric/electrostrictive film is 5 .mu.m at least. That is, it is difficult to form a piezoelectric/electrostrictive film at a thickness less than 5 .mu.m with the conventional printing method.
Second, when a thick piezoelectric/electrostrictive film is formed, it is difficult to realize a high aspect ratio of patterns as well as to align the patterns to the infrastructure. The formation of thick piezoelectric/electrostrictive films is possible by controlling the emulsion film of the screen in use to a proper thickness or repetitively printing ceramic pastes twice or more times. As the printing procedure is repeated, however, newly added ceramic pastes 16 flow down over the side of formerly formed patterns as shown in FIG. 1.
Third, where a thick piezoelectric/electrostrictive film is formed by repetitively printing ceramic pastes, because a thermal treatment is conducted after each printing, the repetitive thermal history may cause the earlier formed ceramic layers to be thermally deteriorated.
Fourth, if the piezoelectric/electrostrictive film is formed in fine patterns, 30 .mu.m or greater must be given to the width of the fine patterns and the distance therebetween. That is, it is quite difficult to obtain a pattern of piezoelectric/electrostrictive films which are finer in dimension than 30 .mu.m by the screen printing method.
Finally, because the consequence of the screen printing is dependent on the screen pattern used and other various factors, such as ceramic paste viscosity, printing pressure, printing speed, etc, account must be taken of such factors.